Normal and genetically engineered skeletal muscle cells (myoblasts) show promise as drug delivery vehicles and as therapeutic agents for treating muscle degeneration in congenital myopathies. Transplanted myoblasts may be used to correct defects in well characterized inherited myopathies such as Duchenne muscular dystrophy. In addition, myoblasts may provide genes to correct myopathies that are not understood at the molecular level. Finally, genetically engineered myoblasts can be used to deliver nonmuscle recombinant proteins to the circulation. Candidates for this type of delivery system include hormones, coagulation factors, growth factors and antitumor agents with broad applications ranging from the treatment of inherited hormone deficiencies to symptoms of aging, Parkinson's disease, heart disease and cancer. Myoblast transplantation poses unique challenges in comparison to organ transplants. In contrast to organs, transplanted myoblasts fuse and become incorporated into the multinucleated muscle fibers of the host. However, the immunobiology of myoblast transplantation is poorly understood. The use of myoblasts from a "universal donor" would greatly increase the general utility of myoblast transplantation. However, allogeneic myoblasts are rapidly rejected unless immunosuppressants are administered. Continuous immunosuppression is associated with toxic side effects. Furthermore, chronic administration of immunosuppressive drugs may affect the clinical course of the muscle disease for which myoblast- mediated gene therapy is intended or affect the fusion of transplanted myoblasts. Alternate strategies of immunosuppression which are transient, but lead to the long term maintenance of allogeneic myoblasts are needed. We have shown that two different transient immunosuppressive treatments result in long term retention of allogeneic myoblasts in mice. The mechanism by which these transient treatments lead to persistent survival of allogeneic myoblasts is unknown. Several mechanisms have been proposed for transient immunosuppressive treatment to prolong allograft survival. These models have different implications for the fate of the original transplanted myoblasts after muscle injury, for the survival of subsequent myoblast transplants and for the formation of stem cells from the transplanted myoblasts. The goal of this research is to determine whether persistent survival of myoblasts with transient immunosuppressive treatment is due to the development of unresponsiveness in the host immune system or instead due to decrease in the expression of molecules involved in allograft rejection by the donor myoblasts. We will also determine if allograft rejection can be blocked by using myoblasts which are genetically defective in lCAM-1. Finally, we will determine if stem cells arise from the implanted cells during transient immunosuppressive treatment. These studies have direct applications to therapeutic approaches to inherited and acquired disease using myoblasts as vehicles for gene delivery.